We are evaluating the mechanism of the blood pressure response induced by osmotic and nomosmotic stimuli of arginine vasopressin (AVP). all experiments are carried out in awake, unrestrained, bilaterally nephrectomized rats. Hypertonic saline administered to the Long Evans strain induced a significant increase in mean blood presure which was not observed in the Brattlebory strain of rat. A dependence on the presence of arginine vasopressin appears to exist . Noloxone, and opiatge antagonist, had no effect on this rise in the Long Evans rats.